Numerous methods exist to measure temperature by using electronic devices such as semiconductors and resistors. Semiconductor devices used vary from a simple, low cost diode to a pair of transistors, specially manufactured for high accuracy of temperature measurement. Other devices use resistors of varying stability to support a highly accurate and easily calibrated temperature measuring device. The resistors are placed in a Wheatstone bridge configuration, A Wheatstone bridge is a common technique wherein a probe, typically a thermistor, causes the bridge to become unbalanced as the temperature changes. The temperature changes sensed by the probe cause the bridge to transmit an analog signal.
Temperature measurement is critical to many applications. Typically, a thermocouple device is insert into a liquid, gas, or in contact with a surface. As the thermocouple senses a temperature change in its environment, the thermocouple circuit creates an analog signal. As opposed to a digital signal, or a series of zeros and ones that represent the state of an object, the analog signal is continuously variable with temperature. However, every analog signal must be converted to a digital signal for use by a computer system, such as a programmable logic controller (PLC) system.
A PLC control system has numerous inputs acting as conditions that a PLC acts upon. Based upon the input temperature, the control system may cause an alarm to sound, or slow the reaction rate through a reduction in a catalyst. It is the critical nature of temperature measurement across numerous applications that result in a wide variety of measuring devices.
A widely used temperature measurement device 100 in FIG, 1A uses a simple, low cost diode in which a constant current IA is applied from a current source. Every diode 104 has a junction voltage with varies with temperature. This voltage shift with temperature is called the slope of the diode, (S). The voltage 106 across the diode is proportional to the temperature of the junction represented by the equation T˜=K−S/degree Celsius. The slope S and the constant K are measured in millivolts. In addition, each diode junction voltage 108 or slope has an error term as the diode heats up because of the constant current source 102.
Other devices use a resistor circuit to adjust the slope of the diodes in the circuit. These resistor circuits are used to compensate for the cold junction temperature of the thermocouple. The cold junction or reference temperature is the analog output of the thermocouple circuit at zero degrees Celsius. A thermocouple device produces an analog output proportional to the measured temperature. Compensating for the cold junction temperature improves the accuracy of the thermocouple device. The analog-to-digital converter reads the voltage difference between the measured temperature and the cold junction temperature. A fixed cold junction signal produces an accurate and repeatable base signal to the analog-to-digital converter. A similar system is described in U.S. Pat. No. 4,441,071, which is incorporated herein by reference. In another system, amplifiers and a power supply are added to the resistor network to compensate for the cold junction temperature. A similar system is described in U.S. Pat. No. 4,126,042, which is incorporated herein by reference.
Other devices use a combination of resistors and amplifiers to measure a voltage change, hence, a temperature change over a predefined range. Typically, these systems are used for measuring high temperatures requiring a highly accurate result. The temperature represented by the output signal is low to moderate requiring the use of amplifiers. A similar system is described in U.S. Pat. No. 5,611,624, which is incorporated herein by reference.
Yet in other devices, the resistor not the diode provides the analog signal based upon the sensed temperature change. The resistors are configured in a Wheatstone bridge circuit with a power source applied across the circuit. A first circuit called a reference circuit provides a first signal that is constant to the analog-to-digital converter. A second circuit called a bridge circuit provides a second signal, the second signal's output is proportional to the temperature change, to the analog-to-digital converter. The comparison of the two separate signals allows the use of inexpensive resistors in the circuit to achieve a highly accurate temperature measurement. A similar system is described in U.S. Pat. No. 5,655,305, which is incorporated herein by reference.
FIG. 1B shows a specially manufactured transistor circuit 150 for measuring temperature. This circuit 150 improves measuring accuracy at a much greater cost than the single diode device 100 of FIG. 1A. A pair of transistors 158 and 160, having a known base-emitter junction area ratio, we inputs to a differential amplifier 166. A voltage 152 is applied across the transistors 158, 160 and a pair of resistors 154, 156 to draw currents 162 and 164 through the transistors. The ratio of the base-emitter current density of the transistor pair 158 and 160 yields a known slope. The error term of the junction voltage at the base emitter cancels out when the current density or surface area of each transistor is controlled at the time of manufacture.
The voltage change due to the measured temperature causes the currents 162 and 164 to flow through the transistor pair 158, 160, which amplify the small voltage change. The amplified voltage across the transistor pair 158, 160 is the input to a differential amplifier 166, having an output proportional to the absolute temperature measured (PTAT). One of the transistors 158, 160 in the transistor pair is the cold junction or reference temperature circuit. A similar system is described in the National Semiconductor LM34, LM82, LM83, and LM84 specifications, which are incorporated herein by reference. A similar system is described in U.S. Pat. No. 4,475,103, which is incorporated herein by reference.
Although some devices use a diode or a group of diodes, each temperature device design trades cost for accuracy and temperature range. The temperature device of FIG. 1A has the advantage of low cost and an output signal capable of being measured without additional circuitry. Additional circuitry increases manufacturing cost, thus, an increased cost to the end product. Moreover, additional circuitry increases cost because of engineering and design considerations in the product using the more complicated circuit.
The diode circuit of FIG. 1A has many disadvantages shared by other low cost circuits. The temperature range and accuracy depends upon the diode type and details of its manufacture. The design engineer using the device of FIG. 1A must have engineering information on the diode itself. Regardless of the diode, the circuit of FIG. 1A has an inherently low accuracy and requires calibration before use. Calibration fixes the use of the FIG. 1A circuit, thus, limiting its reuse in other applications. Last, the higher the temperature measured the lower the voltage output from the circuit. This would require additional circuitry thus leading to an increased cost.
The cold junction or reference temperature circuit used in thermocouple-based devices of other devices leads to additional cost and engineering complexity. Without a cold junction temperature, the actual measurement has no baseline. Additional circuitry provides a cold junction analog signal as an input to the circuitry of the temperature measuring device. The additional circuitry narrows the use of the temperature measuring devices. The narrowed use results from a need to improve accuracy and reduce the calibration time. Electronic circuits lose accuracy because of the heat generated from current used to power the circuit itself. To compensate for lost accuracy, the circuit designers tend to use more expensive electronic components and design for narrower temperature ranges. Designing for narrower temperature ranges allows one to apply the electronic circuit accurately to the narrower range, resulting in a higher resolution much like microscope at high power. The additional circuitry has more stable resistors, power amplifiers, and specially manufactured transistors.
The circuit of FIG. 1B uses a pair of transistors. This circuit has a much higher cost, almost sixty times the circuit of FIG. 1A because of the engineering and manufacturing in the diodes contained in the transistors and the circuit supporting the matched diode pair. Like the diodes of FIG. 1A, the user must know the diode type and the details of its manufacture.
The temperature circuit of FIG. 1B has better accuracy than FIG. 1A and a wider temperature range than FIG. 1A. However, the circuit of FIG. 1B requires careful matching of the diodes used in base emitter of transistors 158 and 160 at manufacture. In addition, more circuitry is required to measure the analog signal proportional to the temperature measured.